1. Field of the Invention
The present invention relates generally to optical devices and components, and more specifically, it relates to filters, integrated filter arrays, and related devices.
2. Description of the Related Art
Multispectral acquisition (MSA) has been widely used in space research and other fields. Wavelength division techniques are most important parts thereof. Conventional wavelength division components include filters rotating in a wheel frame, gratings, prisms, etc. Such components occupy large space and have a large weight which increases the payload of aero crafts. Furthermore, they include mechanical moving parts, which are unreliable and likely lead to overall system failures.
Filter arrays, developed from the 1980s on, are micro-space wavelength division components, which are made up of spectrally distinguishable detectors with detector arrays. This has largely simplified the wavelength division system, and improved the reliability, stability, optical efficiency and signal to noise ratio (SNR). Therefore, the wavelength division systems of new optical apparatuses tend to use such new structures to acquire spectral information. Furthermore, the application of filter arrays improves the integration and miniaturization of relevant sensor devices, and provides powerful support on the study of filter-type micro-spectrometers.
Although filter arrays have a huge application potential, no clear progress has been made for decades restricting their applications. There are two obstacles for the development of filter arrays, namely, micromation and integration. The most perspective integrated filters should be the filters with different spectral characters integrated on a single substrate, whose element size can be in the order of microns and which can be designed and fabricated to match the detector arrays.
There are two kinds of filter arrays integrated on a single substrate. The first kind is realized by fabricating filters conventionally one by one by while masking other areas of the substrate. (See [1] Cheng Shiping, Yan Yixun, Zhang Fengshan, et al., Development of three-channel short-wave IR spectrum distinguishable detector array, J. Infrared Millim. Waves, 13, 401 (1994) (in Chinese); [2] Cheng Shiping, Zhang Fengshan, Yan Yixun, Study on the Technology of Preparing Micro-infrared Filter Array by Masking and Lifting-Off Method, J. Infrared Millim. Waves 13, 109 (1994) (in Chinese)). This is a very complicated fabrication method. The product array rate is one half of the starting array rate when adding an additional filter to the array. For example, provided that the efficiency rate of each fabrication process is 90%, the finished array production efficiency rate is only 0.932 (or about 3%) for fabrication of a 32 channel filter array. Such a low finished product rate leads to very high cost for fabricating integrated filter array by conventional optical thin film techniques, and largely restricts a higher rate of integration.
The second kind of filter arrays integrated on a single substrate is based on the Fabry-Perot interference principle, wherein a high-integration filter array is designed with a spacer layer having a variable thickness which corresponds to the filter's pass band. See, e.g., Chinese Pat. Appl. Publ. No. 200310108346.5.
Spacer arrays with different thicknesses can be realized by using the combinatorial etching technique. The filter array is formed by employing micrometer balls to connect two interference interfaces and control the final thicknesses of the spacer layers. There is merit in controlling the thickness of a spacer array by etching in that an array integrated with 2n filters needs only two times of deposits and n times of etching. For example, only 5 times of etching for fabricating a 32 channel-filter array by using a combinatorial etching technique. The efficiency of this method is 6 times higher than that of conventional methods and the finished product rate is 0.97 (or about 48%), i.e., 14 times higher than that of the conventional method assuming the same product rate for each process. With the increase of integration, the finished product rate will decrease rapidly with an exponential trend by conventional method, while only a little lower than 48% by combinatorial etching technique. The efficiency of this method is much higher than that of the conventional method which has a remarkable advantage on cost saving.
However, the fact that the etching process has been introduced into the fabrication procedure results in two problems. The first problem is the increase in the roughness of the spacer's surface which leads to lowering the property of the filter. The second one is that the control precision of etching does affect the control precision of the filter's passband. Furthermore, it is very hard to control the size of the micrometer balls accurately. Therefore, it is very hard to control the final thickness of spacer layer accurately which in turn leads to the difficulty of controlling the filter's passband accurately.